For the first time ever, scientists have successfully cloned human embryos. Although many species of animals—including mice, cows, cats, mules, and horses—have been cloned, this is the first time the technique has been successful in humans or any primate.

Researchers hope to use the technique, called therapeutic cloning, to produce stem cells that could ultimately be used to replace damaged tissues and organs. Embryonic stem cells have the potential to become any kind of cell in the body. By using a person’s own cells to create the stem cells, any transplanted cells or tissues can avoid rejection by the immune system.

The embryos were allowed to develop to the 32-cell blastocyst stage—the point at which embryos are normally grown before being transplanted during in vitro fertilization procedures. But the goal of the research was not to produce a human baby. Rather, researchers harvested a small number of special cells from the embryos and cultured them to produce embryonic stem cells.

Out of 176 nuclei transferred to donor egg cells, 30 embryos developed to the blastocyst stage. Cells from 20 of these embryos were removed and cultured separately to produce one embryonic stem cell line.

Woo Suk Hwang and his colleagues at Seoul National University in Korea used several tricks to overcome many of the problems that have plagued researchers attempting to clone human cells.

The procedure typically involves removing the nucleus of an oocyte, an immature egg cell, and replacing it with the genetic material from another non-reproductive cell. During most cloning experiments, the egg cell and donor nucleus come from different individuals. But in this case, the egg cells and donor nuclei came from the same women. This may have circumvented any problems due to genetic incompatibility.

Once the nuclei have been transferred to the eggs, the cells must be activated to stimulate development. But before this happens, the new genome has to be reprogrammed: Genes needed by the old cell must be turned off and genes needed by the developing embryo must be turned on.

In typical cloning experiments, the activation step takes place at the same time that the nucleus is transferred. But Hwang and his colleagues waited for two hours before activating the eggs, to allow more time for reprogramming.

In addition, the researchers gently squeezed the nucleus out of the donor cell, instead of sucking it out, to avoid damage to the genetic material.

The researchers then tested the resulting stem cells for their potential to form all the major tissue types in developing embryos by transplanting them into mice. The cells formed the three basic layers of the early embryo and formed a variety of cell types, including nerve, eye, bone, cartilage, muscle, and connective tissue.

Hwang, W.S. et al. Evidence of a pluripotent human embryonic stem cell line derived from a cloned blastocyst. Published online in Science, February 12, 2004.